In satellite communication systems, a transmitting earth station generates a modulated carrier in the form of electromagnetic waves up to a satellite, forming an "uplink." The incident electromagnetic waves are collected by the satellite, processed electronically to reformat the modulated carrier in some way, and retransmitted to receiving earth stations, forming "downlinks." The earth stations in these systems basically consist of a transmitting and/or receiving power station functioning in conjunction with an antenna subsystem and form strategic parts of the satellite communication system.
A TVRO earth station typically comprises a receiving antenna such as a paraboloidal dish, a low noise block conveter (or a low noise amplifier (LNA) and a down converter) located at an outdoor antenna site, and a receiver located near an indoor television set. The down converter and the receiver are usually connected by a coaxial cable.
A single transponder in a satellite can carry a color television channel, including both the video and audio information, and also several auxiliary services such as radio stations, newservice feeds, special news teletypewriter channels, high-speed stock market and commodity exchange data feeds, and/or teletext data services. A transponder normally has a usable modulating signal bandwidth of 8 to 10 MHz, and the video information normally occupies the band up to about 4.2 MHz. The audio portion of the television channel is placed on an FM subcarrier in the 5.8 to 7.4 MHz range (usually either 6.2 or 6.8 MHz), which leaves available all the other FM subcarrier frequencies located above 4.2 MHz. For example, in several of the satellites presently orbiting the earth, transponder owners feed separate audio subcarriers at 5.8, 6.2, 6.8 and 7.4 MHz. Other transponders on the same satellites carry music services at 5.58 and 5.76 MHz.
A satellite transponder may also be used to carry multiple narrow-band audio signals in place of a wide-band video signal. These audio signals may be interspersed with other types of auxiliary services, and thus the exact frequency of the audio subcarriers can vary widely among the large number of transponders presently in orbit, and new subcarriers can become available at any time as more subcarrier services are squeezed onto existing transponders and as additional satellites are placed in orbit. Moreover, there is no fixed relationship among the numerous audio signals themselves.
The particular problem addressed by the present invention is that the numerous audio signals have widely varying bandwidths. Indeed, even the video signal bandwidths can vary; there is at least one satellite that currently transmits video signals in 18-MHz half-transponder format rather than the standard 36-MHz format. These varying bandwidths present a problem in the receiver, particularly in the audio tuner, because the receiver must have a passband wide enough to receive those signals having the widest bandwidths in order to avoid distortion, but then the receiver picks up a large amount of noise with those signals having much narrower bandwidths.
Current TVRO receivers often have a bandwidth adjustment knob which permits the user to select either a "wide" or a "narrow" passband. This is not only a very limited adjustment which often provides much less than optimum reception, but also imposes a bothersome and tedious burden on the user when he is attempting to select from among a large number of satellite signals.